Thank you - that was very useful and solves the iso-boot problem.
I must also boot FreeBSD on NFS via TFTP (not HTTP), but this won't work:
menuentry 'FreeBSD chainload BTX' {
kfreebsd boot/loader }
It's probably because this is a PXE setup and not HDD. If one wishes to chainload to FreeBSD's BTX, I think one would have to actually call boot/pxeboot in order to allow BTX to become aware of the PXE structure.
That means we are left with direct-boot. What are the necessary additions / modifications for below? "192.168.2.1:/data/amd64" is the NFS export which will be mounted as root. TFTP path is "/boot", so any & all files under that folder are visible.
menuentry 'FreeBSD over TFTP & root on NFS' --class freebsd --class bsd --class os {
set kFreeBSD.vfs.root.mountfrom="nfs:192.168.2.1:/data/amd64"